BUENOS AIRES, Aug 27 (Reuters) - Argentine President Mauricio Macri said that monthly inflation would accelerate to 3% in August following a slump in the peso, as the central bank intervened heavily in the market on Tuesday to prop up the local currency.

The peso lost 18% of its value in a week after leftist opposition presidential candidate Alberto Fernandez netted a 15-point lead in an Aug. 11 primary election, as investors reassessed the prospects of business friendly Macri retaining power.

After meeting on Monday with International Monetary Fund officials, who are visiting Argentina ahead of the next review of a $57-billion lending program, Fernandez's leftist coalition issued a strident statement blaming the Fund and Macri's government for the crisis.

On Tuesday, Macri told an agricultural conference that inflation - which was running at a monthly rate of 2.2 percent in July - would tick up to slightly over 3 percent.

"We were going to be at about 1.8% inflation. It's now going to be 3-something in August," Macri said.

Argentina's 12-month inflation is running at 54.4%, according to the national statistics agency. Inflation had been decelerating for the fourth consecutive month in July after reaching a peak of 4.7% in March.

By Tuesday afternoon, the peso had weakened 0.65% to 55.67 per dollar, traders said.

The central bank sold a total of $250 million in reserves on Tuesday, the biggest one-day intervention since the peso started falling on Aug. 12. So far, the bank has sold $1 billion in its own reserves to support the currency against political uncertainty touched off by the primary.

The local Merval stock index fell by 3.4% on Tuesday.

The visiting team from the IMF was expected to meet with central bank officials on Tuesday as part of its review of the standby agreement.

After the Fund's meeting with Fernandez on Monday, his "Frente de Todos" coalition released a statement blasting the agreement.

"Those who have generated this crisis, the government and the IMF, have the responsibility of putting an end to and reversing it," the statement said.

A statement from the IMF on Monday night confirmed the meeting with Fernandez and his team and said it was "a productive exchange of opinions."

Fernandez has pledged to "rework" Argentina's agreement with the IMF if elected, but has so far declined to share details of a plan. (Reporting by Eliana Raszewski; writing by Cassandra Garrison Editing by Alistair Bell)

Our global editorial team of more than 50 journalists and about 250 freelancers covers the world’s under-reported stories at the heart of aid, development, women’s and LGBT+ rights, human trafficking, property rights, and climate change.